Extract from the Chronicle of Bernard the Treasurer accessed and translated from the original Old French text on Google books http://books.google.com/books?id=f-BAAAAAYAAJ&dq=Chronique+d'Ernoul&source=gbs_navlinks_s pp 469-472.
After having told you so much about the land of Outremer, now let us tell you about Constantinople.
The Christians within Constantinople had lost all of their lands except for the city and a little bit of land outside it. They got together and said that a large part of them would abandon the city and go away. Others said that couldn’t bear to do it because of the great shame and the great reproach they would get wherever they went. They would leave such a rich city for nothing. And so, they would send for aid to the Pope and let him know about the state of the land and beg him, by God, to help them and to get them King Jean as their lord. If they could get him, they would get a good grip over their lands with the aid of God. And they would also send messengers to King Jean asking that he come to their land and as soon as he came, they would make him their lord. Everybody agreed to this plan of action. They got their messengers ready and sent them to the Pope and King Jean.
When their messengers got to the Pope and conveyed to him their message, the Pope sent King Jean a message to come talk to him. He came and the Pope told him what people in Constantinople had told him and begged him to do it and agree. The king said that he wouldn’t go. The land already had an heir and he didn’t want to disinherit him. Also he didn’t want to put himself in great danger to save somebody else’s land. The pope begged him to go and promised him lots aid in the form of money and troops. The king said that he wouldn’t go for such a promise, but he wouldn’t refuse the promise if he did go on an adventure. He thanked him.
In the end, King Jean, because he saw how the land needed him and because the Pope was begging him to, said he would go on the condition that the heir-apparent who should be emperor marry his daughter by his Spanish wife wear the crown, if the knights of the land would grant him it and the Pope would bless it. Later, after he would marry his daughter, he would swear on the saints that as long as he lived he would be subject to him and have no power over him. Also, all the knights of his land would pay him homage all his life and any land he would conquer that the emperor’s ancestors had held, would belong to the emperor. As for any land he would conquer, if the emperor’s ancestors hadn’t held it, it would belong to his heirs who would hold it as a fief from the emperor. “If they’ll do that”, said King Jean to the Pope, “I’ll go by the prayer and aid that you’ve promised me. Otherwise, I won’t.” The Pope blessed it. The messengers returned back to Constantinople and said what they heard from the Pope and King Jean. The knights talked together and agreed readily to what the Pope had reported to them.
The people of Constantinople sent messengers again to King Jean and asked that he come to Constantinople. They would do what he had devised. The king, when he heard their message, he went to the Pope and said goodbye to him. The Pope then gave him some money and promised him that he would help him with money and troops if he needed it. Then, the king departed and went to Venice where he embarked and came to Constantinople.
When King Jean arrived in Constantinople, the knights of the land came to meet him and received him very warmly. After he had stayed little bit in Constantinople, he sent for all the knights of the land and he had the young man, who was supposed to be emperor, married to his daughter and made him wear the crown. The emperor and the knights of the land did gladly what King Jean required as they had agreed and the king held them to it.